题解 | #附加题#

#include<stdio.h>
#include<vector>
using namespace std;
const int mod = 1000000007;
int main(){
    int n, i;
    scanf("%d", &n);
    vector<unsigned int> p(n+1), dp(n+2);
    for (i = 1; i <= n; ++i) scanf("%d", &p[i]);
    dp[1] = 0;
    for (i = 1; i <= n; ++i) {
        dp[i+1] = (2*dp[i] - dp[p[i]] + 2 + mod) % mod;
    }
    printf("%d", dp[n+1]);

    return 0;
}